I would have to it all depends on who you are targeting specifically and what irresistible offers you are driving people to land at either the blog/report/case study.

As long as you have a USP (unique selling proposition) that is clearly identifiable and you are able to clearly differentiate yourself from the crowd I feel all could work and depending on what stage of the marketing sales funnel your prospects are in.

Usually if its a new prospect a special report would work as well as case studies. People always want to work with an "expert" as well always looking for someone who they can TRUST.

Hope this help and let me know if you have any other questions as I would be happy to help

1. Social media (depending how many promoters you have)
2. Affiliate Marketing: It is becoming a popular way for others to cheaply promote your business
3. Going to your area's local entrepreneur or start up events

Long answer: It sounds like you're looking at a Content Marketing approach. If that's the case, all three options are basically the same. They are your content. I work with clients who use all three in their strategy. Some just use one or two.

For example, you could offer a free white paper for email signups, have a section on your website for case studies and publish regular blog posts.

Blog posts are the easiest to create, optimize for search and publish, so that's where I would start.

Startups need an strategy which can drive traffic organically else cost effectively.I feel Email marketing is better if compared to other marketing strategies.There are useful tools out there in the market which will help you in making brand name easily.I personally use email marketing technique, for that, i use a tool name Easysendy Pro which helped me in sending bulk emails to generate good business leads with NO SPAM.It is low cost ($23) than any other email marketing tool and also gives you 2000 subscribers free for each month.

All 3 Strategies mentioned are effective in their own manner. Mainly it depends on what type of product you are promoting. It is always better to consult a professional if you have confusion as to which way to follow...! For instance I prefer going for paid marketing.